1. Field of the Invention
The present invention relates, in general, to a method for fabricating a semiconductor device and, more particularly, to use of a mixture phase of ozone gas, anhydrous HF gas and deionized water vapor to control the etch selection ratio between oxide film and polysilicon film.
2. Description of the Prior Art
To etch an oxide film, fluorine compounds are typically used as etchants. For example, an aqueous HF or BOE solution is employed in a wet etch process while CF.sub.4 CHF.sub.3 is used in a dry etch (plasma) process.
For polysilicon film, a mix aqueous solution of fluoric acid, acetic acid and nitric acid may serve as an etchant in a wet etch process. Plasma dry etch is also utilized for polysilicon film.
These conventional techniques are highly selective between oxide film and silicon film. That is, during the etching of an oxide film with such etchants as are used in the conventional techniques, a silicon film is little etched and vice versa.
With such fluorine chemicals, low or similar etch selection ratios are difficult to obtain. Thus, when an oxide or silicon film is needed to a certain extent, a composition suitable to moderately etch them cannot be achieved with such fluorine chemicals.